A device-dependent color space comprises the colors that a device can produce and a color within the space is generally specified by values of a set of color components to which the device is responsive (these components depend on the particular color model being used, for example, the RGB color model is normally used for color additive devices, such as monitors, and has the color components Red, Green and Blue; for color subtractive devices such as printers, the CMYK color model is normally used and has the color components Cyan, Magenta, Yellow and Black).
A device-dependent color space itself is a more or less arbitrary color system until mapped to a reference color space with an agreed color interpretation. Mapping a device-dependent color space to a reference, device-independent, color space results in a definite “footprint” within the reference color space known as a ‘gamut’. The most extreme points in the gamut are termed gamut boundaries. A color in the reference color space that lies outside the gamut boundaries is said to be out of gamut, and cannot be reproduced by the device.
Gamut boundaries are most conveniently defined in the device-independent colour spaces which are uniform. Such spaces must satisfy two main conditions: a) visual difference between two colours can be expressed as a Euclidean distance between two points representing these colours in the uniform colour space, and b) same distances signify the same perceptual colour difference independently of their location within the uniform colour space.
A number of uniform device-independent colour spaces exist. The most widely used ones are those developed and published by the International Commission on Illumination (Commission Internationale d'Eclairage, or CIE). Among the CIE colour spaces, the most common is the L*a*b* (CIELAB). The three coordinates (or dimensions/components) of CIELAB represent the lightness of the color (L*=0 yields black and L*=100 indicates white), its position between red/magenta and green (a*, negative values indicate green while positive values indicate magenta) and its position between yellow and blue (b*, negative values indicate blue and positive values indicate yellow). The L* component closely matches human perception of lightness.
Related to the CIE L*a*b* (CIELAB) color space, is the CIE L*C*h (CIELCH) color space which is a cylindrical representation of the three perceptual color correlates: lightness, chroma, and hue. The axial component of CIELCH is the same lightless attribute L* as CIE L*a*b*, the radial component is the chroma, and the angular component is hue. The transformation of (a*, b*) to (C*, h) is given by:
            C      ab      *        =                            a                      *            2                          +                  b                      *            2                                          h      ab        =          arctan      ⁡              (                              b            *                                a            *                          )            
Other color spaces published by the CIE include colour-appearance spaces CIECAM97 and CIECAM02. While different from CIELAB in properties, all these spaces define similar axes of Lightness, green-magenta and blue-yellow, and have cylindrical representation of Lightness, Chroma and Hue. Other uniform colour spaces are expected to be published in the future.
The majority of image processing algorithms for image enhancement operate on the lightness component of the image modifying this component to effect some desired enhancement, such as local and global contrast manipulations, sharpening, gamma correction etc. For example, contrast enhancement of an image encoded in a device-dependent RGB color space is often carried out as a three-stage process:
1) the image is converted from the device-dependent RGB color space to CIELAB;
2) contrast enhancement manipulations are applied to the lightness L* channel; and
3) the resulting image is converted back to the device-dependent RGB color space.
A problem that may arise, however, is that the lightness contrast enhancement causes pixel colors to drift out of the original device color gamut. In stage 3, as these colors are mapped back to the device-dependent RGB color space, their colors may shift. The direction and the magnitude of the shift depends on the gamut mapping algorithm used for stage 3, and is not controllable by the contrast enhancement algorithm. For example: in the case of standard sRGB transformation the out-of-gamut colors are clipped, resulting in a severe reduction of chroma contrast and loss of Chroma details.